Recently, digitally coded video data has been transmitted by using a network for transmitting data at variable-rate, for example ATM (asynchronous transfer mode). If the network transmits the video data regardless of a number of bits, its transmission delay is low and its picture quality is fixed. But in actual use, the user side (coding apparatus side) declares a parameter value of transmission bits (for example, average bit-rate) to the network side because of traffic control, and the network side monitors whether the user side keeps a declared value or not by using UPC (Usage Parameter Control). Therefore, the user side needs to control the number of bits of video coded data to keep the declared value.
In prior art UPC methods, the use of a sliding window technique is well known as shown in FIG. 1. In the sliding window, a window whose phase is slid by a unit of one frame (or alternatively one field) is set and an average bit-rate in each window is restricted below the declared value.
FIG. 2 shows a block diagram of a prior art variable-rate video coding apparatus for a sliding window. In this apparatus, a number of bits of coded data in the window is summed, and an average bit-rate of coded data is restricted below the declared value by feedback control. In FIG. 2, an input video signal is coded by a coder 1. The coded data from the coder 1 is supplied to a number of bits sum section 2. The number of bits sum section 2 sums the number of bits of coded data in the window. The number of bits information 3 is then input to a variable-rate control section 4. The variable-rate control section 4 compares the number of bits information 3 with a target value and outputs a control information 5. According to the control information 5, the number of bits of coded data from the coder 1 (in short, transmission rate) is variably controlled. In comparison with a video coding method of fixed-rate, a video coding method of variable-rate is effective for transmission because its decoding delay (delay time from input of coding apparatus to output of decoding apparatus) is short. Concrete examples of variable bit rate video transmission are disclosed in Japanese Patent Disclosure (Kokai) No. H2-305284 and IEICE Tech. Rep. IE92-15 pp. 21-27 (May 22, 1992).
A disadvantage in the variable-rate control section of FIG. 2 is that even if the target value (average bit-rate) coincides with the declared value (declared average bit-rate), it happens that the actual number of bits of a particular frame (or field) will be above the declared value. In this case, because of the restriction of the sliding window technique that the average bit-rate in each window is restricted below the declared value, coded data of some frames (or fields) must be transmitted with a low bit-rate. Therefore, as shown in FIG. 3A, it can happen that the number of bits of coded data which can be transmitted fluctuates by a period equal to the window size. To cope with this, there is a method in which a target average bit-rate in a window is below the declared average bit-rate, as illustrated in FIG. 3B. In this method, the actual average bit-rate (possible number of bits of coded data which may be transmitted) is low in comparison with the declared average bit-rate and there is no periodic fluctuation.
Separate from the above, in a predictive coding method of video coding, the number of bits of coded data for each frame is different. FIG. 4A shows a prior art predictive structure for a video coding method used for storage media (VTR etc.). A GOP (group of pictures) consists of a frame of intra coded data (I picture) 41, frames of forward predictive coded data or coded data (P picture) 42, and frames of bidirectional predictive coded data or forward predictive coded data or backward predictive coded data or intra coded data (B picture) 43. As for the picture and B picture, the other kind of frame which is referenced by the P picture and B picture is determined by coding efficiency. In general, the number of bits of each picture meets the relationship "I&gt;P&gt;B". As an example of such video coding method, a standard method of MPEG (ISO/IEC JTC1/SC2/WG11 MPEG 90/176, Coding of Moving Picture For Digital Storage Media) is well known.
The video coding method as shown in FIG. 4A has the drawback that decoding delay is relatively long because the B picture is predicted by reference to a future frame. In contrast, FIG. 4B shows a predictive structure of a frame of forward predictive coding or intra coding from plural past frames instead of a B picture. In this predictive coding method, GOP includes nonreference frames (P' picture) 44 for predictive coding of other frames and decoding delay is low. Therefore, coded data of a video coding method as shown in FIG. 4B is transmitted at a variable-rate whose decoding delay is low.
In the video coding apparatus of variable-rate shown in FIG. 2, it happens that the number of bits of coded data that can be transmitted fluctuates by a period corresponding to window size as mentioned above. Accordingly, for a frame (or field) whose number of coded bits is large, it is necessary that picture quality of the frame is reduced or the frame is skipped because only a small number of bits for the frame can be transmitted.